In time division duplex (TDD) systems concerning 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), when ACK/NACKs (A/Ns) for a plurality of downlink data signals are transmitted through one uplink sub-frame, A/N bundling is used (see FIG. 1). Generally in the LTE, through one downlink sub-frame, a data signal generated using one code word (CW) is transmitted in Non-MIMO transmission, whereas a data signal generated using two CWs is transmitted in MIMO transmission. For these data signals, a terminal generates an A/N for each CW. At this time, in the case of using A/N bundling, the terminal calculates for each CW the logical sum (XOR) of A/Ns of a plurality of sub-frames, to thereby obtain one piece of A/N information (that is, a bundled response signal) for each CW. That is, if even one sub-frame has a NACK in an entire given CW as an A/N bundling target, a NACK is generated as a response signal for the given CW. On the other hand, only if all the sub-frames have an ACK in the entire given CW, an ACK is generated as a response signal for the given CW. That is, focusing on a certain CW, an ACK/NACK is obtained for each sub-frame, and ACK/NACKs are bundled on the basis of the certain CW, whereby one bundled response signal is obtained. Then, the bundled response signal corresponding to each CW is transmitted to a base station (see NPL 1 and NPL 2). The base station performs retransmission control on the basis of the bundled response signal corresponding to each CW. That is, the CW can be regarded here as a unit for retransmitting a data signal. In addition, even in the MIMO transmission mode, a data signal may be generated using one CW. For example, in the case of applying a transmit diversity, a data signal is generated using only one CW.
In addition, before the reception of a downlink data signal, information on resource in which the downlink data signal is to be arranged and the like is reported by a downlink control channel signal (PDCCH) from the base station to the terminal. In order to accurately generate A/N information even if the terminal fails to receive the PDCCH, whether or not a PDCCH for the past sub-frame exists is reported from the base station to the terminal with the use of a downlink assignment indicator (DAI) in the PDCCH. This enables the terminal to determine the number of A/Ns as bundling targets, leading to accurate bundling. In a system not including such a DAI, if the terminal fails to receive the PDCCH, the A/N information cannot be accurately generated in some cases.
In addition, the standardization of 3GPP LTE-Advanced, which achieves a further increase in speed of communication than that of 3GPP LTE, has been started. The 3GPP LTE-Advanced system (hereinafter, may be referred to as “LTE-A system”) follows the 3GPP LTE system (hereinafter, may be referred to as “LTE system”). In the 3GPP LTE-Advanced, in order to achieve a downlink transmission rate of 1 Gbps or higher at the maximum, a base station and a terminal that can communicate with each other at a wideband frequency of 40 MHz or higher are expected to be introduced.
In LTE-Advanced downlinks, carrier aggregation using a plurality of downlink unit bands (component carriers (CCs)) for data transmission is supported. In the case of using such a carrier aggregation technique, an A/N for a downlink data signal of each CC is generated. Accordingly, in uplinks, A/Ns for the plurality of CCs need to be transmitted.